Perturbation of Ikaros isoform selection by MLV integration is a cooperative event in NotchIC-induced T cell leukemogenesis

The chromosomal translocation t(7;9)(q34;q34.3) in human T cell acute lymphoblastic leukemia (T-ALL) results in the aberrant expression of the intracellular domain of Notch (Nic). Consistent with the current multistep model for tumorigenesis, mice that express Nic in T cell progenitors develop a T-ALL-like disease with a lengthened latency. Proviral insertional mutagenesis greatly accelerated the onset of leukemia in Nic transgenic mice. We demonstrate that the Ikaros (Ik) locus is a common target of proviral integration in Nic transgenic mice, which results in the loss of Ik DNA binding activity through altered isoform expression. We propose that cooperative leukemogenesis occurs in cells that have constitutive Nic and altered Ik isoform expression because genes normally repressed by Ik become activated by Nic/CSL.